The invention relates to polystyrene molding compositions and more particularly to polystyrene compositions with high-impact strength produced by a mass-suspension polymerization process. The impact resistance, as well as the tensile and flexural strength, elongation, and bends values of certain polystyrene molding compositions enable broad application and wide usage of these polystyrenes in fields where strength, durability, and ease of manufacture are desired. Accordingly, polystyrenes possessing these properties are conventionally molded in sheet form and structural form, designed and adaptable as packaging structures, housings, support structures, furniture, molded articles, toys, architectural trim, and the like. Greatly improved polystyrenes having high-impact strength, elongation, and bends values are achieved by the discovery of a relationship between the rubber particle size of the styrene copolymer, and the aforementioned physical properties of the composition. It has also been discovered that a relationship exists between the degree of grafting of the rubber particles and the physical properties of the composition, particularly, as in the preferred embodiment of the present invention, when the polystyrene graft copolymers are produced by a mass-suspension polymerization process. Appreciation of these relationships permits realization of the invention by the selection of a specific ratio of an impact-resistant polystyrene having a small rubber particle size and a polystyrene having a large rubber particle size with particular degrees of grafting. Thus, a blended polystyrene composition with surprisingly improved properties such as impact resistance, tensile and flexural strength, high elongation and bends values is achieved.
It is well known that impact-resistant polystyrenes have been prepared by incorporating emulsion polymerized synthetic rubbers in styrene monomer and subjecting the mixture to polymerization. Although the brittleness of the polystyrene material is reduced to a certain degree, by this process, the corresponding increase in impact resistance is generally insufficient in many instances to satisfy the conditions to which a general purpose polystyrene is subjected in common use. Moreover, the improved impact resistance is not achieved without a substantial sacrifice in one or more other, sometimes equally important, properties.
Similarly, it is taught in the prior art that the particle size of the rubber which is incorporated into the styrene polymer, has a high influence on the impact resistance of the composition. For a given rubber content, the impact resistance tested for samples of elastomer particle size varying from 0.5 to 5.mu., pass through a maximum at 1.mu. size.
Compositions of large particles (i.e., 5.mu.) are generally prepared in order to promote properties such as flexibility and elongation.
Molding compositions, in general, have been subjected to many processing changes and variations in molecular weight, rubber content, rubber particle size, and the like, in an attempt to achieve a proper balance of important physical properties. For instance, U.S. Pat. Nos. 3,652,721 and 3,663,656, relating to ABS resins, teach an emulsion polymerization technique capable of producing ABS polymer particles having different sizes in a single polymerization cycle, and a single reactor. ABS resins of improved properties are produced by this single-batch polymerization process. Moreover, U.S. Pat. No. 3,663,656, teaches that in order to improve the physical properties of the polystyrene composition, the small particle graft copolymer must have a superstrate to substrate ratio of from about 45-100:100, and that the large particle graft copolymer must have a superstrate to substrate ratio of 15-40:100. It is further taught that the failure to observe the degree of grafting within these levels precludes improvement of the physical properties of the composition.
Similarly, U.S. Pat. No. 3,592,878, describes a ternary blend of a high cis polystyrene graft copolymer having a rubber particle size of 0.5 to 30 microns, a high trans polystyrene graft copolymer having a rubber particle size of 1,000 to 3,000 A units, and a styrene polymer or copolymer. The high cis large particle graft copolymer is prepared by a mass polymerization process, and the high trans small particle graft copolymer is prepared by an emulsion polymerization process. After the preparation of each component polymer, the two graft copolymers are mechanically blended with the styrene polymer to produce the product composition.
While the processing techniques taught in each of the foregoing patents achieve a certain improvement in the physical properties of the resulting polystyrene compositions, the requirement of an emulsion polymerization process renders each of these processes undesirable in actual use. The large difficulties in recovering the resultant copolymers from the aqueous medium and in controlling the degree of grafting upon the rubber particle while in emulsion discourages the use of processing techniques requiring an emulsion polymerization on a commercial basis.
Other similar examples of processing techniques which have been employed in an attempt to improve physical properties have been described in Japan in patent application Nos. 33,305 and 7,123,145 and in Germany by way of application Nos. 2,101,650 and 2,161,136, also for ABS compositions.